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Physically and chemically synthesized Ti. O 2 composite thin films for hydrogen production by photocatalytic water splitting Supervisor Prof. Antonio Miotello Laboratorio Idrogeno Energia Ambiente Dipartimento di Fisica Facoltà di Scienze MM. FF. NN. Ph. D. Student Rupali Dholam
Current Energy System Fossil fuel reserves such as oil, natural gas and coal have finite reserves and are depleting rapidly. Environmental Damage of Fossil Fuels: Climate Change Ozone Layer Depletion Rains Acid Pollution Air Oxygen Depletion
Fossil Fuel Production/Demand (Petroleum and Natural Gas)
Why Hydrogen Energy? Hydrogen Economy Global , clean and environment friendly permanent energy system……. v The advantages of Hydrogen as fuel are : ü It is the lightest element, and has the highest mass-specific energy content among the fuels: 119. 93 MJ/kg, compared to 44. 5 MJ/kg for gasoline, at present the transportation fuel of choice. ü It is ecologically neutral. ü It is the ideal candidate for use in fuel cells, which produce very little emissions. ü Hydrogen is safer than commonly used natural gas because it mixes much faster with air than either methane or petrol vapors (due to high diffusion coefficient) which make accidents in the open air less critical. ü The major outcome by combustion of hydrogen is water which also contains hydrogen.
But production of H 2 from fossil fuels lead to increase in green house effect.
To make the life cycle of hydrogen fuel to be clean and renewable it is very important to produce hydrogen gas from clean and renewable energy sources such as solar and wind. Transportation H 2 water Primary energy source Energy carrier Electricity generation Energy system consumption water
Catalyst used in Hydrogen production by Water Hydrogen Production: Solar light is used as the energy to required break the water molecule by using Photocatalyst 2 H+ + 2 e- → H 2(gas) 2 h+ + H 2 O(liquid) → 1/2 O 2(gas) + 2 H+ Overall Reaction: 2 hν + H 2 O(liquid) → 1/2 O 2(gas) +H 2(gas) Reaction takes place when the energy of the photons absorbed by the photoanode are equal to or larger than Et, the threshold energy: Et = hν = 1. 23 e. V
Energy Diagram of Photo-electrochemical cell.
Requirement of the photocatalyst: v Must have energy band gap ~ 2 e. V v Must have high Corrosion and photo-corrosion resistance v CB of semiconductor must be more negative than redox potential of H 2 and VB must be more positive than oxidation potential of O 2 v low cost of manufacturing Best Photocatalyst Ti. O 2
Ti. O 2 photo-catalyst thin film is been synthesized on conducting ITO glass by Sputtering (physical method) and Sol-gel (chemical method). Sputtering (physical method) Distance between Ti. O 2 metal oxide target a ITO deposited on glass substrate 5 cm DC Power 150 W Deposition rate 13 nm / min Ar gas flow rate / partial pressure 19 sccm (8 Pa) Total sputtering operating pressure 3 x 10 -5 Pa Substrate heating temperature RT Ti. O 2 ITO Glass Sol-gel (chemical method).
Result and discussion XRD • Debye-Scherrer equation : T =. • Crystal size of anatase phase was 6 nm and of rutile phase was 45 nm. FTIR • sol-gel deposited Ti. O 2 film posses only anatase phase with low crystalline degree However anatase phase is most favorable for photocatalytic reaction.
SEM Sputtered deposited film Sol-gel deposited film • Typically dense columnar structure with diameter around 30 -50 nm is observed in sputtered film • Sol-gel film is quite compact with thickness 135 nm
UV-Visible spectroscopy : • Absorption edge of Ti. O 2 film deposited by sputtering is at higher wavelength (~ 388 nm) • Absorption edge of Ti. O 2 film deposited by sol-gel is at wavelength (~ 370 nm) • The energy band gap of chemically prepared sample is 3. 4 e. V which is higher than theoretical value for anatase (3. 2 e. V) and rutile (3. 0 e. V) • Sputtered deposited Ti. O 2 band gap is 3. 21 e. V. The absorption edge also contains shoulder at 2. 85 e. V indicating presence of impurity energy level in the band gap • Band gap is obtained by fitting absorption edge of UV-Visible spectra by following equation ln T = ln T 0 - C
Photocatalytic Activity. Different Thickness of ITO (nm) Photo-Voltage measured in distilled water (volt) Sputter deposited Ti. O 2 (1000 nm) Solgel deposited Ti. O 2 (100 nm) light off light on 30 0. 130 0. 686 ----- 50 0. 170 0. 720 0. 122 0. 558 100 0. 196 0. 583 ----- 150 0. 246 0. 577 0. 170 0. 553 250 0. 140 0. 543 0. 159 0. 649 350 ----- 0. 134 0. 707 • In case of sputtered deposited film Voc shows maximum value when placed on thinner ITO films as compared to thicker films. • Since the conductivity is inversely proportional to the thickness of ITO film, thus deposition of Ti. O 2 on thinner ITO films (30 and 50 nm) gives better electrical contact and favoring better photo-voltage. • Thickness of ITO film can be decreased further to enhance the photo-voltage value but this will result incomplete coverage of the substrate.
Composition analysis along the cross-section of the sol-gel deposited Ti. O 2 thin film (a) before and (b) after heat-treatment at 500 o. C. • Reverse behavior is observed for sol-gel deposited thin film which showed increase in Voc by increasing ITO thickness up to certain value. • Heat treatment on sol-gel film causes diffusion of Ti atoms into ITO layer up to depth of 120 -150 nm that will change the peculiar properties of ITO which results in low Voc for thinner thickness. • For thicker ITO film (250 and 300 nm) the Ti atoms partially diffused in to the ITO film thus preserving its properties and shows better Voc.
Electrical contact Apparatus to measure Separate evolution of H 2 and O 2
Hydrogen measurement • The H 2 generation rate for sputtered deposited sample for sol gel film it was 4. 3 ± 0. 1µmole/h. Ti. O 2 was 12. 5 ± 0. 1µmole/h and • Due to band gap (3. 2 e. V) , impurity level contributed by stoichiometric defect, the sputtered deposited Ti. O 2 film leads to higher production of H 2 than sol-gel film.
v Conclusions ü Two different kinds of Ti. O 2 films were prepared using RF sputtering and the other one by sol–gel method for hydrogen production by water splitting in photoelectrochemical cells. ü Depositions were performed on electrical conducting ITO whose electrical properties play vital role to reduce the photon energy loss. ü The photo-anodes(Ti. O 2) have been characterized by several techniques to infer on their optical and compositional properties. ü The observed differences in hydrogen production have been attributed to the peculiarities in absorption properties of the two Ti. O 2 films that in the case of sputterdeposited films are more prone to absorb radiation because of the produced defects during the deposition process. Publication : Physically and chemically synthesized Ti. O 2 composite thin films for hydrogen production by photocatalytic water splitting. R. Dholam, N. Patel, M. Adami, A. Miotello , International Journal of Hydrogen Energy, 33 (2008) 6896 -6903.